1. Field of the Invention
The present invention relates to a dynamic convergence apparatus provided in a color cathode ray tube.
2. Description of the Prior Art
In an inline 3-beam system color cathode ray tube (hereinafter simply referred to as a color CRT), if electron beams are converged at the central portion of a picture screen thereof when a deflection magnetic field of a deflection yoke is a uniform magnetic field, then a dynamic convergence of a longitudinal arch type is generated at the upper and lower and right and left porions of the picture screen as shown in FIG. 1. FIG. 1 of the accompanying drawings shows such dynamic convergence thus generated and in which R (red) and B (blue) represent beams of the respective sides and G (green) represents a central beam. Therefore, there have heretofore been proposed dynamic convergence apparatus which carry out the dynamic convergence by largely distorting a deflection magnetic field of the deflection yoke (barrel magnetic field and pin-cushion magnetic field). According to the conventional dynamic convergence apparatus, however, a beam spot of an electron beam is distorted at the peripheral portion of the picture screen of the color CRT and a focusing is deteriorated.
To remove the aforesaid drawbacks, there is proposed a dynamic convergence apparatus having a quadrupole coil provided at the rear stage of a deflection yoke in which a convergence current is flowed to the quadrupole while the deflection magnetic field of the deflection yoke remains as the uniform magnetic field. In this case, however, the deflection magnetic field is still distorted and the distortion of the beam spot of the electron beam on the peripheral portion of the picture screen of the color CRT cannot be removed completely. Further, this apparatus is not suitable for a multi-scan monitor because a current flowed to the quadrupole coil is controlled. Furthermore, in this apparatus, waveforms cannot be fine controlled without difficulty and the convergence cannot be fine adjusted on a very small region of the picture screen.
As a dynamic convergence apparatus of special use, there is proposed a method in which a convergence voltage modulated by an external transformer is supplied to convergence plates of an electron gun via a coaxial cable. Although this conventional dynamic convergence apparatus can improve convergence at the peripheral portion of the picture screen of the color CRT to some extent, this apparatus needs a high voltage transformer so that this apparatus becomes expensive and cannot be applied to consumer color television receivers.
Further, there has heretofore been proposed a dynamic convergence apparatus in which resistors of high resistance value are connected to a pair of inside electrodes and a pair of outside electrodes within the color CRT, capacitors are formed by forming conductive layers on the inner and outer surfaces of a tube envelope of the color CRT and a convergence voltage is applied to a pair of outside electrodes provided within the CRT from the outside of the tube envelop through capacitors.
A conventional dynamic convergence apparatus (see Japanese published patent publication No. 55-20633) will hereinafter be described with reference to FIG. 2. In FIG. 2 reference numeral 22 generally denotes an overall arrangement of an electron gun apparatus of a Trinitron (registered trademark) bipotential type in which three electron beams of red, green and blue are positioned in line.
Cathodes 23R, 23G and 23B for red, green and blue electron beams are provided within the horizontal plane in line. Common first to fourth grids 12, 13,14 and 15 are arrayed sequentially on the central axis of the envelope, in that order. Convergence means 5 is provided at the rear stage side of the fourth grid 15. Second and third grids 13, 14 constitute a common pre-focus electronic lens for the red, green and blue electron beams R, G and B. Third and fourth grids 14, 15 constitute a common main electronic lens. The red, green and blue electron beams R, G, B are crossed at substantially the center of the main electronic lens by the pre-focus lens and then diverged. The center electron beam G passes through a space between a pair of high voltage side electrode plates 1 and 2 of the convergence means 5. The red electron beam R passes through a space between the high voltage side electrode plate 2 and a low voltage side electrode plate 4. The blue electron beam B passes through a space between the high voltage side electrode plate 1 and a low voltage side electrode plate 3.
A capacitor 7 is formed by forming conductive layers 7A, 7B of semi-annular shapes on the inner and outer surfaces of the tube wall of a neck portion 6N of a tube envelope 6 at its position where the convergence means 5 is provided. An insulating layer 24 made of a material such as ceramics or the like is stretched across the fourth grid 15 and the low voltage side electrode plate 3. A resistance coating film 25 is coated on one surface of the insulating plate 24 at its one half portion at the fourth grid 15 side. Electrodes 26, 27 are attached to both ends of the resistance coating film 25 to form a resistor 28 having a high resistance value. Another capacitor 32 in such a manner that electrodes 30, 31 made of a stainless steel are soldered to both surfaces of a dielectric thin member 29 made of barium titanate by silver is attached to one surface of the insulating plate 24 on its another half portion. A conductive member 33 is also attached to the electrode 26 of the high resistance value resistor 28 by welding. The conductive member 33 is attached to the fourth grid 15 by welding. A conductive leaf spring 34 is stretched across the electrode 27 of the high resistance value resistor 28 and the electrode 30 of the capacitor 32. A free end of the conductive leaf spring 34 is brought in contact with the inside conductive layer 7A, and a conductive member 35 is attached to the other electrode 31 of the capacitor 32 by welding. The conductive member 35 is attached to the low voltage side electrode plate 3 by welding.
The high voltage side electrode plates 1 and 2 are connected via a conductor 10, and the electrode plate 2 is connected to the fourth grid 15 by a conductor 11. An elongated portion 16a of an inside conductive layer 16 deposited on the inner surface of the funnel portion of the tube envelope 6 is formed within the neck portion 6N. A conductive resilient contact member 17 is attached to the fourth grid 15 so as to contact with the elongated portion 16a, whereby an anode voltage applied to the inside conductive layer 16 from the external conductor of a coaxial anode button attached to the funnel portion of the tube envelope 6 is supplied to the fourth grid 15 and the high voltage side electrode plates 1, 2.
A conductive semi-annular shaped plate spring 18 is inserted into the neck portion 6N of the tube envelope 6 at its end portion of the funnel portion side. A conductive resilient contact member 19 is attached to the low voltage side electrode plate 4 and is brought in contact with the plate spring 18. Then, the convergence voltage (which will be described later on) that results from dividing a high DC voltage by a resistor is supplied to the inside conductor through a core of a coaxial anode button. The convergence voltage is supplied to the low voltage side electrode plates 3, 4 by connecting the inside conductor to the plate spring 18 through a conductor 21 covered with an insulating tube 20. While the capacitor 7 is formed on the neck portion 6N at its portion near the funnel portion as shown in FIG. 2, the capacitor 7 may be formed on the neck portion 6N at its portion near the center around the elongated portion 16a of the inside conductive layer 16. Also in this case, the conductors 7A, 7B constructing the capacitor 7 cannot be deposited in perfect annular shape and deposited in semiannular shape such that they are prevented from contacting with the elongated portion 16a.
The example of the conventional dynamic convergence apparatus shown in FIG. 2 will further be described with also reference to its equivalent circuit shown in FIG. 3.
As shown in FIG. 3, a convergence voltage (dynamic convergence voltage) e.sub.c1 supplied from a convergence correction voltage signal generating source 36a having an inner resistor 39 is supplied through the capacitor 7 formed by utilizing the tube envelope 6 and the capacitor 32 within the tube envelop 6 to the low voltage side electrode plates 3, 4. On the other hand, an anode voltage Eb is directly supplied to the high voltage side electrode plates 1, 2 and to the inside conductive layer 7A through the resistor 28 within the tube envelope 6. In this case, because the capacitor 32 is provided, the anode voltage Eb is prevented from being supplied to the low voltage side electrode plates 3, 4. The low voltage side electrode plates 3, 4 are supplied with a convergence voltage Ec that results from dividing the anode voltage Eb by the resistors 37, 38. In FIG. 3, reference numeral 40 designates a coating capacitance. The capacitance of the capacitor 7 is selected to be in a range of from about 100 pF to 200 pF when a color CRT has a diameter of 29 mm at the neck portion of the tube envelope 6. A withstand voltage thereof must be selected to be about 2 kV. A resistance value of the high resistance value resistor 28 is selected to be in a range of from about 30 M.OMEGA. to 100 M.OMEGA..
However, the conventional dynamic convergence apparatus thus arranged encounters the following drawbacks:
When the convergence voltage is supplied from the outside to the inside through the capacitor 7 which is provided by depositing the conductive layers 7A, 7B on the inner and outer surface of the tube envelope 6 of the color CRT, if the convergence voltage is obtained by effecting the amplitude modulation by the parabolic wave of horizontal and vertical period, a low frequency component, i.e., vertical parabolic component cannot be transmitted. For this reason, the component that results from effecting the amplitude modulation only by the horizontal parabolic wave is supplied to the inside from the outside through the capacitor 7 that is obtained by depositing the conductive layers 7A, 7B on the inner and outer surfaces of the tube envelope 6 of the color CRT as the convergence voltage. The vertical parabolic wave component is supplied to the deflection yoke or quadrupole. Such arrangement is complicated from a system standpoint and is not so effective in improving a defocusing of an electron beam at the peripheral portion of the picture screen of the color CRT.
This will be described more. Assuming that the capacitance of the capacitor 7 and the resistance value of the high resistance value resistor 28 shown in FIGS. 2 and 3 are 50 pF and 50 M.OMEGA., then a time constant .tau. is expressed as: EQU .tau.=50.times.10.sup.-12 .times.50.times.10.sup.6 =2.5 msec
Although the alternating current signal of horizontal period can be transmitted because the time constant .tau. is sufficiently long as compared with a period 63.5 .mu.S of a horizontal deflection frequency 15.75 kHz, an alternating current signal (vertical parabolic component) of vertical period cannot be transmitted unless the resistance value or capacity is made about 15 times larger because the vertical deflection frequency is 60 Hz (16.7 msec).
The capacitor 7 of such large capacity is not advantageous in actual practice in use. If a resistor having a larger resistance value is used as the high resistance value resistor 28, then a voltage fluctuation of several Volts is generated even by a very small current of several 10 s of nanoamperes and a convergence error occurs. Accordingly, a limit of the time constant .tau. is several microseconds.